Ion intensity control mechanism for mass spectrometers



y 1952 M. G. INGHRAM ET AL 2,602,898

ION INTENSITY CONTROL MECHANISM FOR MASS SPECTROMETERS Filed June 50, 1950 5 Sheets- Sheet l IN VEN TOR. Mark 6. bay/7rd & By Brice M Puszad fiwyu rwaaw MTTONV July 8, 1952 G. INGHRAM ET AL ION INTENSITY CONTROL MECHANISM FOR MASS SPECTROMETERS 3 Sheets-Sheet 2 Filed June 50, 1950 INVENTORS d g h. @Md w m Z W awn y 1952 M. G. INGHRAM ET AL 'ION INTENSITY CONTROL MECHANISM FOR MASS SPECTROMETEIRS Filed June 30, 1950 5 Sheets-Sheet 5 9 u H f v v 8 v d U [1% M \INVENTOR. Mark 6. flag/7r y Brice M (.1

am & side! I I 75 Vo/zfaye Source A 7'TOENEY Patented July 8, 1952 ION INTENSITY CONTROL MECHANISM FOR MASS SPECTROMETERS Mark G. Inghram, Chicago, 11]., and Brice M. Rustad, New York, N. Y., assignors to the United States of America-as represented by theUnited States Atomic Energy Commission Application June 30, 1950, Serial No. 1713651 12 Claims.

Our invention relates to mass spectrometers and more particularly to an arrangement for automatically controlling the ion intensity thereof to facilitate continuous and rapid analysis of gases.

Heretofore, the mass spectrometer has been utilized as a continuous gas analyser. It has been the practice tocontinuously sample a gaseous mixture in a system. A sample is customarily bled from the system and continuously fed to the source of a mass spectrometer where it is subjected to electron bombardment. As a result of this bombardment, ions of the components of the mixture are formed in proportion to the relative concentrations of such componentsin the sample. The ions are then separated according to their masses, and the respective ion currents are measured. From the relative magnitudes of these currents the quantitative composition of the mixture can be determined.

In general, the intensity of the ion beam does not stay constant, but varies with changing spectrometer source conditions and with the pressure of the sample gas. When this intensity variation is rapid as compared with the time required for analysis, a greatamount of care must be exercised by the mass spectrometer operator in both readjusting the ion intensity, and in collecting a sufiiciently large quantity. of data, in such a manner as to statistically minimize the errors caused by the variation in intensity.

Applicants with a knowledge of these problems in the prior art have for an object of their invention the provision of a mass spectrometer for continuously sampling and analysing gases by automatically controlling the ion intensity to overcome the necessity for time consuming manual adjustments and operations.

Applicants have as another object of their invention the provision of a mass spectrometer for continuous analysis, arranged to control the ion intensity of the ion beam through regulation of the flow of gas into the mass spectrometer by making such flow responsive to the ion intensity of the beam to be controlled.

' Applicants have as another object of their invention the provision of a mass. spectrometer having a valve for regulating'the flow of gases to be analysed, and an anti-hunt arrangement for limiting the action of the valve to insure uniform flow of gases upon adjustment.

Applicants have as a further object oftheir-in vention the provision of a mass spectrometer for continuous analysis of gases with an arrangement for automatically preventing changes in the flow of the gases to be analysed while the background readings are being taken.

Applicants have as a still further object of their invention the provisionof a mass spectrometer for the continuous analysis of gases with an arrangement for automatically preventing changes in the flow of gases to be analysed when the sample is beingpumped out of the spectrometer.

' Other objects and advantages of our invention will appear from the following specification and accompanying drawings, and'the novel features thereof will be particularly pointed out in the annexed claims. a

In the drawings, Fig. 1 is a schematic of our improved system for controlling the ion intensity of a mass spectrometer. Fig. 2a is a schematic of the circuits of the intensity selector and the motor controller'of our improved system. Fig. 2b is a schematic of the circuits of the range speed control, power supply and automatic shut-off control of our improved system. Fig. 3 Ban elevation, partly in section, of the anti-hunt drive and gear shift mechanism for our improved systerm. I

The control method of the present invention is based on an automatically controlled by-pass valve which is located in the sample lead to the spectrometer tube. The automatic intensity control unit is shown in the block diagram in Fig. 1.

The spectrometer tube l, shown in that figure,

is of conventional design such as the type. used in the analysis of uranium hexafiuoride, separating the ions into U F5+ and U F5+. The collector 2 collects the U F5+ ions and the slit plate 3 collects the U F5+ ions. The ion currents are preferably fed into separate highly stable degenerative feed-back amplifiers A, A, that is, the currents from the plate 3 of the mass spectrometer l are fed into amplifier A while the currents from plate 2 thereof are fed into amplifier A. Their respectiveoutputs are then-fed to conventional recording instruments at K. The amplifiers in question are of the D. C. type conventionally employed to amplifythe output of a mass spectrometer, and may take the general form of that disclosed in the prior co-pending application of Brenholdt, Ser. No. 21,022, filed -April 14, 1948, now Patent No- 2,541,198. However, as an alternative, the ion currents from the mass spectrometer I may be fed into a dual feed-back amplifier instead of the single amplifiers shown in Fig. 1. The dual amplifier may be, of conventional designwhere the ion currents pass through separate grid resistors to ground thereby setting up two separate voltages which are dependent on the ion currents. These voltages are independently amplified and then are transmitted to suitable voltage recording instruments and. from the values of these output voltages, the analysis of the gas is indicated. This amplifier is designed with as much negative feed-back as possible, in order to make the amplifier extremely stable and as far as possible independent of any fluctuating conditions which might occur in the electronic elements within the feed-back amplifier. 7

The control circuit may be made to keep constant either of the ion currents or the sum total of both ion currents. In the example being de- CPI scribed, it was decided to keep the U F5+ ion beam constant. To do this the output voltage indicating the presence of U F5+ was fed into the intensity selector, D of Fig. 1. The intensity selector is set at a value that produces the value of voltage which bucks ,out the desired value of U Fs+ ion beam. Thus, if the U F5+ ion beam is greater than desired the output of the circuit, being the algebraic difference between the output of the dual feed-back amplifier and the intensity selector, would indicate a voltage of a specific polarity. If the ion beam is smaller than is desired the output of this circuit wouldbe of opposite polarity. If the ion beam is at the desiredintensity the output of this circuit is a null v t e- The output voltage from selector D is fed into a motor controller J of a conventional type, such as the Brown motor controller,'manufaotured by the Brown Instrument Company of Philadelphia.

The D. C. output from the intensity selector D is v first converted to an alternating current of a particular frequency, and then amplified in the motor controller J and finally applied to one winding of an appropriate motor G. The control motor G may be a conventional two phase induction motor, sometimes called a split phase induction motor. One phase can be continuously excited from a 110 volt A. C. line while the other phase is excited from the audio frequency amplifier of the motor controller unit J. The direction A of rotation, of such a motor depends upon the' time phase relation between the fixed phase voltage and control phase voltage. The motor G is geared to the throttle by-pass valve B in Fig. 1. The connection to the windings of the motor G are chosen so that if the intensity of the ions impinging upon the ion collector is greater than the desired intensity, as determined by the adjustment of selector D, the motor rotates in a direction which opens the by-pass valve, and accordingly greater portions of the sample are shunted away from the mass spectrometer I. When the ion intensity falls below that determined-by the setting of the selector D, the motor reverses direction, thereby permitting a greater fiow of the sample'to, pass into the spectrometer. This process comprises the regulating action of the automatic ion intensitycontrol unit.

- Due to time lags between the change of sample fiowat the by-pass valve B and the corresponding change in ion intensity, large oscillations can de in the circuit, by this time the valve would be open to the point where an excess amountof gas would be in the tube. and as a result the ion beamwould continue to increase to the point where the valve would start to turn in the opposite direction, and if the time delay were sufliciently large, large oscillations would result rendering the. system useless. In order. to overcome this tendency, a range speed control circuit E and an anti-hunt system, shown more in detail in Fig. 3, were provided.

This system utilizes a range-speed control electronic circuit E and a gear shift solenoid F. The range speed control circuit E excites the gear shift solenoid F whenever the ion intensity is not within a given range; thus, when the ion beam intensity is very nearly at the desired value the gear shift solenoid is not excited and the by-pass valve B is rotated at about 10 R. P. M. Whenever the ion beam intensity changes by more than a certain specified value, voltage is applied to the solenoid F, which shifts gears and changes the speed of rotation of the by-pass valve B to R. P. M. Therefore, for large changes of ion beam intensity the speed of the'valve adjustment will be large enough to bring the system rapidly to within the range of the desired beam intensity, and then the valve speed will decrease to an extremely slow speed, thereby preventing any possible oscillations from occurring.

To provide additional simplicity of operation two other modifications were incorporated into the electronic circuits by means of the automatic shut-01f circuit H of Fig. 1. .This circuit accomplishes twopurposes: (a) Prevents the by-pass valve from turning to the end of its stroke when the sample is being pumped out of the mass spectrometer tube. (b) Prevents the valve from being turned whenthe peaks in-out switch is thrown.

Referring now. to Figs. 2a and 2b which constitute one circuit diagram and show certain of the circuits used in our improved system, 4 designates a voltage divider or potentiometer of the intensity selectorv D, which is connected across the output ofa conventional amplifier A, referred to above, that amplifies the U F5+ ion intensity. A center tap of the voltage divider 4 is connected to a source of bucking voltage including battery or other F. source 5 across which is bridged a second potentiometer 6. The output of this intensity selector then feeds into a vibrating voltage converter 1 of the controllerJ, preferably of the standard Brown motor controller type, where it is converted into A. crpotential of a selected frequency, such-as 60 cycles. The converteris energized from a secondary winding of power transformerfil and the output is passed through an input transformer Band is amplified by conventional resistance "coupled audio frequency power amplifier 9, [0, H and I2 fed by the conventional power transformer 8| from line 2?.

The output of this amplifier of the motor-controller J is then applied through leads l3 and 14 to one phase of'the motor G. Plates of tubes 9, [0. .are' suppliedby a separate secondary of transformer 8| fron'i'that employed to supply the plates of tubes 1 I, 'l2. The filaments of the tubes may be supplied from a third secondary ina conventional manner.

The range of slow speed operation'is limited by the setting of the potentiometer I5 of the range speed control E shown in Fig. 2b. Since potentiometer I5 is coupled through condenser It to the output of the first stage9 of the amplifier ofthc motor controller J, it is apparent that the voltage across the potentiometer is that of the output of the first stage9 of the amplifier. A portion of-this voltage, depending upon the setting of the potentiometer I5, is fed directly to thecontrol ridof a triode l1 and is amplified by it. Energizing potential for triode I1 is supplied by a powerpack of conventional form through load resistor 3|. The power pack includes power transformer connected' to power line 22, double -wave rectifier 26, filter -21, VR tubes 29, 29,

and resistor 30. I r

The output of triode I1 is impressed through resistors I9, l9 and condensers l8, 18 upon the control grids of thyratrons 20, 20, which are normally biased to cut-ofi by E. M. F. sources 23, 23.

22. When the ion beam intensity increases, the

voltage output of the triode' I! increases, and at a predetermined magnitude, fires the thyratrons 20, 20 which act as electronic switches applying line voltage from the output of transformer 21 to the gear shift solenoid F through lines I3, 24, for operating it in the manner heretofore described.

The peaks-in-out switch-32 may be employed to deflect the ion beam of the spectrometer away from the collector plates 2, 3 by an electrostatic held in a conventional manner (not shown). when the background readings are to be taken. Deactivation of the motor G, duringthis period, can be accomplished by relays 33, 34, a triode and a pentode 36. The current for energizing the relay 34 is that which flows through the output circuit of triode 35. When switch 32 is closed, external voltage is applied through line 31 to the winding of relay 33 to energize it, causing the armature to move to front contact position, completing a circuit from ground through the front contact and armature to the control grid of tube 35. This operates, in view of the positive potential on the cathode, to reduce the conductivity or render the tube non-conductive, deenergizing the winding of relay 34 in the output circuit thereof, and permitting its armatures to fall to back contact position. This opens the power circuit through lines [4, 38 to the motor G, and serves to deactivate the motor. When the switch 32 is again opened, permitting the beam of the mass spectrometer to return to the collector plates 2, 3, the winding of relay 33 is dc-energized and the armature returns to back contact position. However, by virtue of the time lag introduced by the large resistor 40, preferably of one megohm, and

the condenser 4|. preferably of 1 mfd, tube 35 does not immediately commence to conduct and relay 34 remains tie-energized. This is due to "the action of the resistor-capacitor combination which prevents the grid 35 from changing po-' tential immediately, and delays its return to normal conduction. This in turn delays the operation of motor G and prevents the by-pass valve B of Fig. 1 from changing its setting until the collector plates of the mass spectrometer are fully charged and the output of the feed-back amplifier A has returned to normal. Further control is realized by connecting the control grid of the tube 36 through line 39 to the output of amplifier A for the application of potential therefrom to the tube, so that when the potential of this grid is raised to a predetermined value by an increase in the output of amplifier A, the plate current of tube 36 increases, thereby causing a large voltage drop across the load resistor 42, of preferably 150,000 ohms in the plate circuit :of the tube. asince the control grid of tube 3.5 is coupled thereto, this action will have the effect of reducingtheconduction of tube.35 to' a point at or near cut-offs This drop in fiowof current across tube 35 will serve to tie-energize thewinding of relay 34 and permit the armatures thereof tofall to back contact position, and .openthe power circuit to. the motor-GI The stopping of motor Gthus prevents the by-pass valve B from' being turned to the end of its stroke when gas pressure is reduced toalow value; v .Inconnectionwith the above circuit, it will'be understood that power for energizingithe plates of tubes 35; 36 is supplied through line 43 and switch 44 from the conventional power supply, referred to above. Appropriate heating current for the filaments for these tubes is supplied from a secondary winding 45 on transformer 25 through switchv 44 as indicated at MN. Bias for tube 36 is regulated by inserting an adjustable cathode resistor 46 in the cathode circuit of the tube between thecathode and ground. Potential .for the screen grid of pentode 36 and for raising the potential of the cathode of. tube 35 to an appropriate biasing is supplied through a line from the juncture of VR tubes 26,29 of the power pack.

The valve which controls the input flow of gas by shunting part of it away from the mass spectrometer I may be similar to that set forth in the co-pending application of Herod, et al., Ser. No. 651,284, or it maytake any other suitable form for the purpose. I v Now referring to Fig. 3 showing one form of gear shift and drive mechanism. used in our improved system for preventing hunting, F represents generally the solenoid for shifting the gears. It includes an outer coil 41 with an axial bore therethrough. Positioned in the bore is an armature or plunger 48 which is slidable axially or longitudinally of the bore. Italso serves as a shaft for rotation, having a recess 49 in one end for the reception of a ball 50 therein. A cupped shaped member 5| is joined through a shank or extension tothe ball 50 and serves to urge the ball against the plunger through the action of coil spring 52, having one end thereof seated in the cupped shaped member. The opposite end of the spring is seated in an adjustable cupped shaped retainer 53, having an adjusting screw 54 passing therethrough and secured to or engaging the end of spring 52. The adjusting screw 54 is maintained in adjusted position by a lock nut 55. The retainer 53 is, in turn, held in place by cross member 56 which receives it and which is joined to the solenoid case by screws 51, 51, or other appropriate means. The solenoid means F is secured to bracket 58 which is in turn mounted on frame 59 by adjusting studs or screws 60. Also mounted on the frame 58 is the driving motor G having a shaft 6| upon which is mounted driving gears or pinions 62 and 63 in spaced relation. These gears are permanently meshed with gears 64 and 65 which are rotatably mounted on plunger 48 and shaft 66. Shaft 66 is connected to and serves to drive by-pass valve B to closed position. One end of shaft 66 is enlarged and receives the reduced end of plunger 49 which is journaled in a conventional bearing in th frame and is connected to shaft 66 through key 61.

Mounted on shaft 66 and plunger 48 at their juncture is a collar 68 which serves as one part of a positive duplex clutch. The collar or clutch element is also secured in place on the shaft 86 and theplunger 48 bytheends of key 61 whichpass through "openings therein. The collar or clutch lar 68 is interlocked with gear 64, it will be understood that the shaft 66'will rotate atrelatively highspeeds, due to the. lower ratios of gears .62, 64.. However, when solenoid F is de-energized, the spring 52 acting through ball 50 will force the plunger. 48. outwardly or longitudinally to the right, disengaging collar 68 from clutch element 15 of gear 64 and bringing it into engagement with the clutch element '16 carried by gear 65. As pins 69, 10 seat in openings 12, 'H of the clutch element 16, the collar or clutch element 68 becomes interlocked with the clutch element 16 of gear 65. In this relation, the shaft 66 is locked to. and rotates with gear 65, which, due to the relatively high ratios between gears 63 and 65, rotates at relatively lower speeds than when locked-to rotate with gears 62 and 64.

Having thus described our invention, we claim:

1. A control systenrfor an ion source comprising a mass spectrometer having an ion source and collector, an inlet for feeding vapors to said source, and means responsive to the flow of ion current. to the collector of said spectrometer for regulating vapor flow to said source.

2. A control system for an ion source comprising-a mass spectrometer having an ion source and ccllector,-an inlet line for. feeding sample gases to said. source, a valve in said inlet line, and means responsive to ion current at the collector for actuating the valve to control the flow of gases through said line.

3. A control system 'for an ion source comprising a mass spectrometer having an ion source and a collector, an inlet line for feeding a gaseous mixture to said source, .a valve interposed in said inlet line for regulating the flow of said gaseous mixture to the source, means for actuating said valve to open and closed position, and

means responsive to ion current at the collector for controlling the operations of the actuating means.

4.. A control system for an ion source comprising a mass spectrometer having an ion source and a collector, an inlet line for feeding sample gases to said ion source, a valve interposed in the inlet line for controlling theflow of. gases to the source, a motor for actuating the valve to open and closed position, and means responsive to ion current at the collector for controlling the operation of the motor.

5. A control system for an ion source comprising a mass spectrometer having an ion source and a collector, an inlet'for feeding-gaseous mixtures to the source, and means responsive to large-changes in ion current for rapidly adjusting the flow of said mixtures to said source and to small changes in ion current for slowly adjusting the flow of said mixtures.

.6. A control system for an ion source comprising a mass spectrometer having an ion source and a collectonan inlet line forfeeding a gaseous imixture to said source, a valve in said inlet line,

and means responsive to, large changes of-ion current at said collector .for rapidly ,.,adjusting said valve and to small changes of ion current atthe collector for slowly adjusting. said valve to provide closeregulation of, the flow; of ,said mixture. i 1

'7. A control system for an ion source comprising a mass spectrometer having an ionzsource and a collector, an inlet line for feeding a gaseous mixture to said source, a valve in said inlet line for regulating the flow of said mixture in said line, means for actuating-said valve toopen and closed position, and means responsive to large changes ofion current'at said collector for controlling said actuating means'to rapidly actuate said valve and to. small changes of ion currentat said collector forcausing Said actuating means to slowly actuate said valve.

8. A control system. for an ionsource comprising a mass spectrometer having an ion source and a collector, an inlet line for feeding sample gases to said ion source, a valve interposed in the inlet line for controlling thefiow of gases to the source, a motor for actuating the valve to open and closed position, and .means responsive to large changes of ion current at the collector for controlling the operation of the motor to rapidly actuate the valve and to smallchanges ofJion current for controlling the operation .of the mo- .tor to slowly actuate said valve.

9.. A control system for an ion source comprising a mass spectrometer having an ion source and a collector, an inlet for feeding vapors to. said source,.means responsive to the flow of .ion current to the collector of said spectrometer for regufor preventing movement of said valve...

'11. A control system for an ion source comprising a mass spectrometer having an ion source and a collector, an inlet line forfeeding a gaseous mixture to the source, a valve interposed in said inlet line for regulating the flow ofgsaid gaseous mixture to the source; means for actuating said valve to open and closed position, means responsive to ion current at the collector for controlling the operation oiv the actuating means, and means responsive to cessation of the flow of ion. current for rendering said actuatingmeans inoperative.

12. A control system for an ion source comprising a mass spectrometer having an-ionsource and a' collector, an inlet line for ifeeding sample gases to said ion source, a valve interposed'in the inletline for controlling the flow of gases to the source, a motor for actuating the valve to open and closed position, meansresponsive to ion current at the collector for controlling the operation of the motor, and means responsive to the cessation of ion current v flow for rendering said motor inoperative. I 1 I G. INGHRAM.

BRICEJVI. BUSTAD.

- No references cited-i 

